Yes, scientists really did bring light to a halt for about a minute. In 2013 a team at the Technische Universität Darmstadt in Germany stored a pulse of light inside a specially prepared crystal for just over sixty seconds and then released it again, with the information it carried still intact. At the time it was the longest light storage reported in any system, and it was a genuine step toward the memory chips that future quantum networks will need.
"Freezing" is a vivid word for it, but the light did not turn into ice or sit still like a parked car. What the physicists actually did was hand the light's information over to the atoms of a crystal, keep it safe there, and then turn it back into light on demand. Below we explain how that works, why anyone would want to do it, and how the record has been pushed much further since.
What the Darmstadt team achieved
The experiment was led by Thomas Halfmann with Georg Heinze and Carsten Hubrich, and it was published in Physical Review Letters in July 2013 under the title "Stopped Light and Image Storage by Electromagnetically Induced Transparency up to the Regime of One Minute". The medium was a small crystal of yttrium silicate in which a tiny fraction of the yttrium atoms are replaced by praseodymium ions. The crystal was cooled to cryogenic temperatures, close to absolute zero, to keep its atoms as quiet as possible.
To show that the stored light still carried usable information, the team did not just store a plain flash. They imprinted a simple picture on the beam, three horizontal stripes about 100 micrometres in size, stored it for more than a minute, and read it back out. The retrieved image was recognisable, which proved that the crystal had kept not only the light's energy but also its spatial pattern.
There was a catch, and the researchers were open about it: the efficiency was low. Less than one per cent of the original light came back out after the longest storage times. For a proof of principle that was acceptable, but it showed how much engineering would be needed before such a memory could be practical.
Why stopping light is so hard
Light in a vacuum travels at 299,792,458 metres per second. In a single minute it covers roughly 18 million kilometres, or about 11 million miles. That is enough to go from the Earth to the Moon and back more than 23 times. Nothing we can build can simply catch a photon and hold it in a box, because any mirror or wall either absorbs it or bounces it onward.
The trick is to stop thinking of light as something to trap and start thinking of it as information to transfer. Light can be slowed when it passes through a material, because it interacts with the atoms along the way. In work published in Nature in 1999, a team led by Lene Hau at Harvard famously slowed a light pulse to about 17 metres per second in an ultracold gas of atoms, slower than a cyclist. Her group and others later found ways to stop a pulse altogether and restart it. Each advance relied on the same basic idea that the Darmstadt team used.
How the trick works: electromagnetically induced transparency
The key effect is called electromagnetically induced transparency, or EIT. Normally, praseodymium-doped crystal absorbs light at certain frequencies. But if a second, stronger "control" laser shines on it at just the right frequency, quantum interference cancels that absorption over a very narrow band. For a short window, the opaque crystal becomes see-through to the signal beam.
Inside that narrow transparent window, the signal pulse slows down dramatically. Now comes the clever part. If the control laser is switched off while the pulse is inside the crystal, the transparency disappears and the light's information is written into the atoms as a collective pattern known as a spin wave. The photons are gone, but a precise imprint of them remains in the quantum states of millions of ions. Switch the control laser back on and the process runs in reverse: the spin wave is converted back into a pulse of light that continues on its way.
The figure above, from the published study, shows the arrangement of energy levels used (a) and how the stored signal faded with storage time under different conditions (b). The small striped images along the top are the retrieved pictures after 0.1, 1, 10 and 60 seconds.
Keeping the atoms from forgetting
Storing light for microseconds is one thing; holding it for a minute is another. The stored imprint fades as the ions are disturbed by their surroundings, including fluctuating magnetic fields from neighbouring atoms. To fight this, the Darmstadt group used a set of radio-frequency pulses, a technique called dynamical decoupling, that repeatedly flips the spins so that random disturbances largely cancel out. They also tuned the magnetic field around the crystal and used computer optimisation to find the best settings for the lasers. Together, these steps stretched the storage time from seconds to about a minute.
How big a leap was it?
In a commentary for the American Physical Society's Physics magazine, Hugues de Riedmatten of the Institute of Photonic Sciences (ICFO) in Barcelona put the result in context. Before this, single photons had been stored in atomic gases for around ten microseconds, and classical light pulses had been held for about 16 seconds in a cloud of ultracold atoms confined by an optical lattice. In solid crystals, storage beyond one second had already been reported. De Riedmatten wrote that the Darmstadt result outperforms earlier demonstrations in atomic gases by about six orders of magnitude
, a comparison that refers to the earlier single-photon storage times.
He also highlighted something easy to miss: because the crystal stored a whole picture, a memory like this could in principle hold many quantum bits side by side, each in a different "pixel". That kind of parallel storage, called multiplexing, matters a great deal if quantum memories are ever to carry real amounts of data.
Why quantum memories matter
Ordinary internet traffic can be copied and amplified as many times as needed, which is why signals can cross oceans. Quantum information cannot be copied in the same way, a basic rule of physics known as the no-cloning theorem. Photons sent through optical fibre are gradually lost, and you cannot simply boost them with an amplifier without destroying their quantum properties.
The proposed solution is a chain of quantum repeaters: stations that catch quantum states, hold them until a link further along the chain is ready, and then pass them on. That waiting step requires a quantum memory, and the longer and more reliably it can hold information, the better the network can work. Long storage could also make quantum-secured communication over large distances more realistic, and it is a building block for some designs of optical quantum computers.
So when headlines in 2013 said scientists had "frozen" light, the real story was this: a memory had been shown to hold an optical signal, including an image, for a time long enough to be useful in such systems.
The record kept moving: from a minute to an hour
The one-minute mark did not last long as the record. In 2021 a group at the University of Science and Technology of China, led by Li Chuanfeng and Zhou Zongquan, reported in Nature Communications that they had stored light coherently for more than one hour. They used a different method, an atomic frequency comb memory in a rare-earth-doped crystal, combined with a carefully chosen magnetic field and dynamical decoupling. After an hour in storage, an interference test showed the retrieved light had kept its phase information with a reported fidelity of 96.4 per cent.
The Chinese team suggested that memories lasting hours could one day be physically carried by aircraft or high-speed train, acting as a kind of "quantum flash drive" for moving quantum information between distant places. That remains a long-term goal. Efficiency, noise and the need for deep cooling are still serious obstacles, and research groups around the world continue to work on them.
Common misunderstandings
- "Light was frozen solid." No. The light pulse was converted into an atomic excitation and later converted back. During storage there was no moving light inside the crystal at all.
- "This breaks the speed of light." It does not. Slowing or storing light inside a material says nothing about the speed of light in a vacuum, which remains the cosmic speed limit.
- "Quantum computers are now just around the corner." The experiment was an important step for one component, the memory. Practical quantum networks need many other pieces, and low efficiency was a major limitation of the 2013 result.
What makes this story worth remembering is how it shows physics turning a poetic idea, catching a beam of light, into careful engineering. A crystal, two lasers and a lot of patience were enough to make light wait a whole minute, and that minute opened the way to an hour.
Sources: G. Heinze, C. Hubrich and T. Halfmann, "Stopped Light and Image Storage by Electromagnetically Induced Transparency up to the Regime of One Minute", Physical Review Letters (2013) | Hugues de Riedmatten, Physics (APS) Viewpoint | Nanowerk: Scientists store light for over one hour (2021) | Wikipedia: Slow light | Wikipedia: Quantum memory | Earlier coverage: io9
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YEAH SCIENCE BITCH!
ReplyDeleteLight is not the fastest thing in the universe, it is not!
ReplyDeletethe fastest thing in the universe is thought.
;~)
No bad news is faster.
DeleteOnly 300 million meters per second, and 20 roundtrip to the moon? Come on, that's so stone age. We're in the 21st century, can't we make a breakthrough in doubling the speed of light?
Delete